The present invention relates to a charging device employing a magnetic brush which charges uniformly an image-forming object in an image forming apparatus such as an electrophotographic copying machine or the like.
Heretofore, there has generally been used a corona charging unit for charging an image-forming object such as a photoreceptor drum or the like in an image forming apparatus of an electrophotographic type. In the corona charging unit, high voltage is applied to a discharge wire around which a strong electric field is formed to cause gaseous discharge, and charged ions produced by the gaseous discharge are adsorbed on the image-forming object, thus the image-forming object is charged.
Such conventional corona charging unit employed in an image forming apparatus has an advantage that an image-forming object is not damaged when it is charged because it can be charged without touching mechanically the corona charging unit. However, the corona charging unit is disadvantageous in that there is a risk of electric shock and electric leakage due to high voltage used therein, and ozone produced in gaseous discharge is harmful for the human body and shortens life of the image-forming object. Further, the charged voltage produced by the corona charging unit is unstable because it is highly affected by temperature and humidity, and the corona charging unit requires some time to obtain a stable charging voltage after initial inputting of high voltage, which are serious problems when an image forming apparatus of an electrophotographic type is used as a communication terminal or an information processor.
Many disadvantages of a corona charging method mentioned above are caused by gaseous discharge accompanying the charging.
In this connection, as a charging device capable of charging an image-forming object without requiring gaseous discharge as in the corona charging unit and without giving mechanical damage to the image-forming object, there is disclosed a charging device in Japanese Patent Publication Open to Public Inspection No. 133569/1984 (hereinafter referred to as Japanese Patent O.P.I. Publication) wherein a magnetic brush formed by adherence of magnetic particles on a cylinder holding therein a magnet can brush, for charging, the surface of an image-forming object.
However, even in the case of the charging device disclosed in the aforementioned Japanese Patent O.P.I. Publication No. 133569/1984, it has been impossible to charge an image-forming object uniformly with perfect stability.